Since portable electronic devices such as mobile phones, portable radios, and PDAs are designed for the purpose of portability, the devices are reduced in overall size or thickness. Therefore, the speaker devices used in such portable electronic devices also are reduced in size or thickness. Generally, the minimum resonance frequency f0 of a speaker device are reduced to obtain an acoustic characteristic with small distortion over a wide frequency bandwidth.
To meet the demand for a reduction in thickness or size of such speaker devices, it is thought to reduce the weight of a diaphragm that vibrates in response to an electric signal applied to a voice coil and emits a sound wave (hereinafter referred to as “an acoustic wave”), the weight of an edge portion that is attached to the circumferential edge of the diaphragm to support the diaphragm, and the weight of other components. For example, the weights of the diaphragm, edge portion, and other components can be reduced by decreasing the thicknesses thereof.
However, when the thicknesses of the diaphragm, edge portion, or other components are reduced, these components are easily deformed, and a reduction in rigidity is, of course, caused. The rigidity is a physical quantity related to the resistance to deformation of a structural body. When the rigidity of the diaphragm, edge portion, or other components is small, a rolling phenomenon, a split vibration (split resonance), or other phenomenon is likely to occur. This results in, for example, an increase in incidental noise, occurrence of an unusual sound, and sound distortion, causing a problem in that good sound quality cannot be obtained.
Now, the rolling phenomenon means that the vibrating system of a speaker device does not linearly move up-and-down in an emission direction of an acoustic wave (a vibrating direction of a voice coil) in response to the electric signal applied to the voice coil and vibrates in a direction substantially perpendicular or oblique to the emission direction of the acoustic wave. In addition, the split vibration (split resonance) is a phenomenon that the diaphragm is bended and different parts of the diaphragm thereby vibrate differently. Furthermore, the split resonance is the following phenomenon. Vibrations created by the vibrational movement of the voice coil bobbin propagate concentrically from a central portion of the diaphragm toward a circumference of the diaphragm thereof. Then the vibrations are reflected from the edge portion and propagate in the reverse direction from the circumference of the diaphragm toward the central portion. The vibrations reflected from the edge portion and subsequent vibrations propagating from the voice coil bobbin interfere with each other to cause the split resonance.
Therefore, a vibrating body for an acoustic transducer that has the following structure has previously been proposed to improve the rigidity of the edge portion included in the vibrating body for an acoustic transducer. More specifically, in this vibrating body for an acoustic transducer, a dome-shaped diaphragm is formed integrally with an edge portion disposed on the outer circumference thereof, and the edge portion includes a groove-shaped rib formed integrally therewith. In addition, an adjustment member that partially improve the bending strength of the edge portion is disposed on a part of the front or rear surface of the edge portion (see, for example, Patent Document 1). Hereinafter, this art is referred to as a first conventional example.
To provide a speaker device that can be reduced in size without an increase in the lowest resonance frequency f0, a vibrating body for an acoustic transducer that has the following structure has previously been proposed. More specifically, in this vibrating body for an acoustic transducer, a first vibrating part that functions as a diaphragm is disposed at the center; a connection part to which a voice coil connects with the outer circumference of the first vibrating part; and an edge portion is disposed integrally on the outer circumference of the connection part. In addition, a second vibrating part that functions as a diaphragm is disposed on the outer circumferential side of the connection part so as to be continuous with the edge portion (see, for example, Patent Document 2). Hereinafter, this art is referred to as a second conventional example.
Moreover, a vibrating body for an acoustic transducer that has the following structure is proposed, the vibrating body can has a sufficient rigidity over the entire area of a dome-shaped diaphragm and reduce a fluctuation in a high tone frequency characteristic caused by a harmonic distortion and reproduce a sound in a high quality, even when dimensions of the dome shape is large. That is, in this vibrating body for an acoustic transducer, a dome-shaped diaphragm is supported by a frame via an edge portion formed integrally with the outer circumference of the diaphragm. The edge portion has, on its outer circumference, reinforcing ribs having convex/concave structure. The diaphragm has a groove- or a ridge-shaped reinforcing rib that is formed radially from the center of the dome so as to extend from the vicinity of the center of the dome toward the outer circumference of the dome (see, for example, Patent Document 3). Hereinafter, this art is referred to as a third conventional example.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-048494 (claim 1, [0011], [0019] to [0025], FIGS. 1 and 2)    [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-166070 (claim 1, [0011], [0017] to [0025], FIGS. 1 and 2)    [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-287418, (claim 4, [0013], [0015] to [0020], FIGS. 2 and 3)